Stainless Steel Pipe Fittings

Quality inspection of welding elbow

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The weld of the welded elbow must be inspected, which is also an important measure to ensure the quality of the weld. After the completion of the elbow welding, the weld joint shall be inspected according to the technical requirements of the product. Any defects that do not meet the technical requirements shall be repaired in time (such as direct disposal that cannot be repaired if it is seriously unqualified). Common inspections of welding quality include visual inspection, non-destructive testing and mechanical performance testing. These three are complementary to each other, and they are mainly based on non-destructive testing.
1. Visual inspection: generally based on the naked eye observation, sometimes with a 5-20 times magnifying glass for observation. Through visual inspection, surface defects of weld elbow welds, such as undercuts, welds, surface cracks, pores, slag inclusions, and weld penetration, can be found. The dimensions of the weld can also be measured using a weld detector or template.
2. Non-destructive testing: Inspection of defects such as slag inclusions, pores and cracks hidden inside the weld. At present, the most common use is X-ray inspection, as well as ultrasonic flaw detection and magnetic flaw detection. The X-ray inspection uses X-rays to photograph the weld seam, and judges whether there are defects, the number and type of defects in the interior based on the image of the film. Then, according to the technical requirements of the product, the weld is qualified. The basic principle of ultrasonic flaw detection is shown in the figure below. The ultrasonic beam is emitted by the probe and transmitted to the metal. When the ultrasonic beam is transmitted to the metal-air interface, it is refracted and passed through the weld. If there is a defect in the weld, the ultrasonic beam is reflected to the probe and accepted, and a reflected wave appears on the screen. Based on the comparison and discrimination of these reflected waves with normal waves, the size and position of the defects can be determined. Ultrasonic flaw detection is much simpler than X-ray photography and is therefore widely used. However, ultrasonic flaw detection often can only be judged based on operational experience, and can not leave a test basis. For internal defects that are not deep from the surface of the weld and extremely small cracks on the surface, magnetic flaw detection can also be used.
3. Hydraulic test and air pressure test: For pressurized containers requiring sealing, hydraulic pressure test and/or air pressure test shall be carried out to check the sealing and pressure bearing capacity of the weld. The method is to inject 1.25-1.5 times working water or a working pressure gas (mostly air) into the container for a certain period of time, then observe the pressure drop in the container and observe whether there is leakage outside. According to these, it can be assessed whether the weld is qualified.
4. the mechanical performance test of the elbow: non-destructive testing can find the inherent defects of the weld, but can not explain the mechanical properties of the metal in the heat affected zone of the weld, so sometimes the welded joint should be subjected to tensile, impact, bending and other tests. These tests were performed by the test panels. The test panels used are preferably welded together with the longitudinal joints of the cylinder to ensure consistent construction conditions. The test panels were then tested for mechanical properties. In actual production, only welded joints of new steel grades are generally tested in this respect.
Source: China Stainless Steel Elbows Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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